Re: Enabling peer to peer device transactions for PCIe devices

[Logan Gunthorpe]

On 25/11/16 06:06 AM, Christian König wrote:
> Well Serguei send me a couple of documents about QPI when we started to
> discuss this internally as well and that's exactly one of the cases I
> had in mind when writing this.
> 
> If I understood it correctly for such systems P2P is technical possible,
> but not necessary a good idea. Usually it is faster to just use a
> bouncing buffer when the peers are a bit "father" apart.
> 
> That this problem is solved on newer hardware is good, but doesn't helps
> us at all if we at want to support at least systems from the last five
> years or so.

Well we have been testing with Sandy Bridge, I think the problem was
supposed to be fixed in Ivy but we never tested it so I can't say what
the performance turned out to be. Ivy is nearly 5 years old. I expect
this is something that will be improved more and more with subsequent
generations.

A white list may end up being rather complicated if it has to cover
different CPU generations and system architectures. I feel this is a
decision user space could easily make.

Logan